The present invention is hydrophobic silica gels having reduced surface area and a method for their preparation under neutral conditions. The method comprises two steps, where in the first step the pH of a mixture comprising a silica hydrosol and a colloidal silica is adjusted with a base to within a range of about pH 3 to pH 7 to form a silica hydrogel having incorporated therein colloidal silica. In the second step the silica hydrogel is contacted with an organosilicon compound in the presence of a catalytic amount of a strong acid to effect hydrophobing of the silica hydrogel to form a hydrophobic silica gel having a surface area within a range of about 100 m.sup.2 /g to 450 m.sup.2 /g in the dry state. In a preferred method the hydrophobic silica hydrogel is contacted with a sufficient quantity of a water-immiscible organic solvent to convert the hydrophobic silica hydrogel into a hydrophobic silica organogel. The water-immiscible organic solvent can then be removed from the silica organogel to form a hydrophobic silica gel having a surface area within a range of about 100 m.sup.2 /g to 450 m.sup.2 /g in the dry state. A water soluble compound of cerium or iron may be added during conduct of the second step to improve the heat stability of the hydrophobic silica gel.
Although hydrophobic silica gels prepared by the present method are useful in many applications such as thermal insulating, reinforcing and extending filler in natural rubbers, and as filler in floatation devices, they are particularly useful as reinforcing fillers in silicone rubber compositions. It is well known that silicone rubber formed from the vulcanization of polydiorganosiloxane fluids or gums alone generally have low elongation and tensile strength values. One means for improving the physical properties of such silicone rubber involves the incorporation of a reinforcing silica filler into the fluid or gum prior to curing. However, silica reinforcing fillers have a tendency to interact with the polydiorganosiloxane fluid or gum causing a phenomenon typically referred to as "crepe hardening." A great deal of effort has been made in the past to treat the surface of reinforcing silica fillers with organosilanes or organosiloxanes to make the surface of the silica hydrophobic. This surface treatment reduces or diminishes the tendency of the compositions to crepe harden and improves the physical properties of the cured silicone rubber.
Brown, U.S. Pat. No. 3,024,126, teaches a method for making a pre-formed reinforcing silica filler hydrophobic by treating it in an organic solvent with an organosilicon compound, such as an organosilane or low-molecular weight organosiloxane containing 0.1 to 2 total hydroxyl and/or alkoxy radicals per silicon atom, and a small amount of amine, quaternary ammonium, or organometallic compound.
Lewis, U.S. Pat. No. 3,979,546, teaches a method for making the surface of reinforcing silica fillers hydrophobic through the use of alpha-alkoxy-omega-siloxanols with alcohols under mild conditions. The fillers taught are pre-formed solids.
Tyler, U.S. Pat. No. 3,015,645, teaches the making of hydrophobic silica powders by reacting an organosilicon compound such as dimethyldichlorosilane or trimethylmethoxysilane with a silica organogel in the presence of an acidic catalyst and then removing the volatile materials. The method requires the preparation of a silica hydrogel which is converted to a silica organogel by replacing the water in the silica hydrogel with an organic solvent.
Lentz, U.S. Pat. No. 3,122,520, teaches a procedure where an acidic silica hydrosol is first heated to develop a reinforcing silica structure and then mixed with an organosilicon compound, an acid catalyst, and a water-immiscible organic solvent to produce a hydrophobic silica filler. The organosilicon compounds taught by Lentz are limited to those compounds in which the organic radicals bonded to silicon atoms have less than 6 carbon atoms, organosilicon compounds having no organofunctional substituents bonded to silicon atoms, and to organosilicon compounds having no hydrogen bonded to silicon atoms.
Alexander et al., U.S. Pat. No. 2,892,797, describe silica sols modified by treatment with a solution of a metalate so that the silica particles are coated with no more than a molecular layer of a combined metal which forms an insoluble silicate at a pH between 5 and 12. Aluminum, tin, zinc, and lead are taught as the preferred metals. Alexander et al. teach that silica sols which carry a metal upon the surface of the particles according to their invention have increased stability at pH extremes.
Termin et al., U.S. Pat. No. 3,850,971, and Termin et al. U.S. Pat. No. 4,006,175 teach that porous silicic acid having a specific surface area of about 50 m.sup.2 /g to 1000 m.sup.2 /g can be made by hydrolyzing methyl or ethyl silicate or polymethyl or polyethyl silicate with about 70 to 120% of the stoichiometric amount of water with moderate stirring. Termin et al. teach that transition metals such as iron oxides and chromium oxides may be used as hydrolysis activators and that such metals may appear in the end product.
Nauroth et al., U.S. Pat. No. 4,360,388, teach cerium containing precipitated silica. Nauroth et al. teach that silicone rubber compositions reinforced with the cerium containing precipitated silica exhibit excellent heat stability and that the cerium containing precipitated silica acts as a fire retardant agent.
Nauroth et al., U.S. Pat. No. 4,208,316, teach the use of hydrophobic precipitated silica as a reinforcing filler in plastic masses which are hardenable to form elastomers. Such elastomers include silicone elastomers.
Jansen et al., EP 0-690-023 A2, teach the formation of silica gels by adding base to an aqueous waterglass solution and aging of the silica gels at a pH of 4 to 11 prior to a hydrophobing step. The described gels do not contain colloidal silica.
Jensen et al., EP 0-658,513 A1, teach the formation of silica gels by adding base to an aqueous waterglass solution and aging of the silica gels at a pH of 6 to 11 prior to a hydrophobing step. The described gels do not contain colloidal silica.
The neutral conditions of the present method for preparing the colloidal silica-containing hydrogels provide advantages over a process where the silica hydrogel is prepared under strong acid conditions. The advantages include reduced use of acid in the process, the ability to use less acid resistant process equipment, and faster conversion of the colloidal silica-containing silica hydrosol into the corresponding silica hydrogel.